FALLUJA, Iraq - Insurgents in Iraq have added water to their arsenal of weapons after seizing control of a dam in the west of the country that enables them to flood certain areas and prevent security forces from advancing against them.

The dam helps distribute water from the Euphrates river on its course through the western province of Anbar, and is located some 5 km south of the city of Falluja, which was overrun by militants early this year.

Iraqi troops have since been surrounding Falluja and shelling the city in an effort to dislodge anti-government tribes and insurgent factions including the Islamic State in Iraq and the Levant (ISIL).

In February, ISIL took control of the Nuaimiya area where the dam is located, and began fortifying their positions with concrete blast walls and sand bags, according to anti-government tribesmen who said no other groups were involved in the takeover.

The militants closed all eight of the dam's 10 gates one week ago, flooding land upstream and reducing water levels in Iraq's southern provinces, through which the Euphrates flows before emptying into the Gulf.

Anti-government tribal fighters said ISIL's tactic was to flood the area around the city to force troops to retreat and lift the siege on Falluja.

"Using water as a weapon in a fight to make people thirsty is a heinous crime," said Oun Dhiyab, a government adviser to the water ministry. "Closing the dam and messing with Euphrates water will have dire consequences."